Yes it is:
Harley mount adapter
Sierra MC 800 338-6938
www.sierra-mc.com SIE-HARLEY-ADAPT $10.00
Now be aware, it is not a strong adapter configuration, the problem is not the adapter but the chromed brass stud that HD uses for their antenna mount. Don't be mistaken, it is not Chrome Steel. The chrome brass stud is ok as long as you use a medium spring from Firestick, tighten it securly, do not overtighten as you will break it. NO LOCKTITE, it is electrical & locktite will insulate the connection & kill the CB Signal.
Firestick Spring SS-3M stainless medium $9.99. If you don't use this spring the HD stud will break. Do not use their stiff spring.
Don't ask me how I know that!!
After the 1st stud broke

I made one out of a 5/16" grade 8 bolt, cut the head off, driled a hole in the the bolt to match the stud that attaches to the antenna wire & and got a 5/16" adapter at Home Depot, drilled & tapped one side for 3/8" to match the Firestick.
A little work, but if you want the best CB performance at it's maximum it is worth it Buy the Non Ground Plane Firestick, with the Non Ground Plane cable. Then make your own adapter & using an SWR meter you should be able to tune to 1.5 to 1 or less.
Do I still run this? No, I hated the look of the Firestick, I now run a cable matched HD short antenna, that was really a challenge & no, it does not work as well as the ugly Firestick. (Ugly on a CVO, ok on a vehicle)
I have 2 firestick antenna's, I will sell them cheap, no cables, you would have to get that from Firestick. They sell them seperate.